1 Field of the Invention
This invention relates to the synchronous data transmission systems which use double sideband-quadrature carrier (DSB-QC) modulation and, more particularly, to a method and a device for quickly determining the initial values of the coefficients of a complex transversal equalizer.
Generally, the invention proposes a method of determining the initial values of the coefficients of a complex transversal equalizer wherein the training sequences consist of periodic pseudo-random sequences of complex numbers having a periodic autocorrelation function, all the coefficients of which except the first are zero, the amplitude of all complex numbers of the sequence being constant.
2. Description of the Prior Art
In the synchronous data transmission systems, the bit sequence or message to be transmitted is first converted into a sequence of symbols each of which can take on a discrete number of values that is generally equal to a power of two. The symbols are then sent over a transmission channel at a given rate, called signaling rate, in the shape of pulses that may or may not be modulated depending on whether or not the transmission system uses a carrier modulation technique. Generally, the transmission channels, and more particularly the telephone lines, introduce amplitude and phase distortions that modify the shape and the signals. These distortions are generally due to the imperfect transfer characteristics of the transmission channels involved and are aggravated by the noise introduced therein by external sources, which are more or less difficult to control. The amplitude and phase distortions can create an interaction between successive signals. This interaction, known as intersymbol interference, may preclude reliable detection of the data by the receiver. In high speed data transmission systems, the receivers are generally provided with a device designed to minimize the effects of intersymbol interference before the data are detected. Such a device is called an equalizer.
The type of equalizer which is the most widely used at the present time is the automatic transversal equalizer described, for example, in "Principles of Data Communication" by R. W. Lucky, J. Salz and E. D. Weldon, Jr., Chapter 6, McGraw-Hill, New York, 1968. Such an equalizer consists of a transversal filter whose coefficients are automatically adjusted to meet a given performance criterion. In general, during an initial period referred to as a training period, a set of isolated test pulses or a periodic pseudo-random binary training sequence is transmitted to allow the equalizer coefficients to be adjusted to initial values that are as close as possible to optimum values. At the end of the training period, the initial values will remain unchanged during transmission of the message, if the equalizer is not of the adaptive type; alternatively, they can be adjusted at any time during transmission of the message, if an adaptive equalizer is used.
If the distortion characteristics of the transmission channel vary between successive messages, which is the case where the channel consists of public telephone lines, a training period must be provided before transmission of every message. However, the efficiency of a data transmission system is generally determined by working out the ratio of the time required for transmitting the message to the occupancy time of the line, the latter time corresponding essentially to the training period plus the message transmission time. If the efficiency level is to remain suitably high in high speed data transmission systems, in which a message can be transmitted in a few tens of milliseconds, provision must be made for a device capable of minimizing the duration of the training period, that is to say, of determining as quickly as possible the initial values of the equalizer coefficients.
U.S. patent application Ser. No. 701,730 filed by the present applicant July 1, 1976, discloses a method which allows the initial values of the coefficients of a transversal equalizer to be quickly determined. In this method, the training sequences consist of periodic sequences of pseudo-random real numbers having an autocorrelation function, all the coefficients of which are zero except the first coefficient, the sequences being derived from periodic pseudo-random binary (1,0) sequences exhibiting particular properties. While this method yields very good results, it has been found that it could be improved to meet the requirements of those synchronous data transmission systems which use DSB-QC modulation. DSB-QC modulation is a class of modulation techniques that includes phase shift keying (PSK), amplitude phase shift keying (A-PSK), and quadrature amplitude (QAM) modulation. The three techniques exhibit similarities and can often be dealt with as a single technique as it appears, for example, from chapters 7 and 9 of the above-mentioned book by R. W. Lucky et al., or from an article entitled "Adaptative Maximum-Likelihood Receiver for Carrier-Modulated Data Transmission Systems", by G. Ungerboeck, in IEEE Transactions on Communications, Vol.COM-22, No. 5, May 1974, pp.624-636, or again from an article entitled "Simultaneous Adaptive Estimation and Decision Algorithm for Carrier Modulated Data Transmission Systems", by H. Kobayashi, in IEEE Transactions on Communication Technology, Vol.COM-19, No. 3, June 1971, pp.268-280. One of the characteristics of DSB-QC modulation is that the data symbols are complex numbers. Thus, it is desirable to use training sequences which exhibit the same characteristics as the data symbols. Also, the systems which employ DSB-QC modulation generally use a so-called complex transversal equalizer, the coefficients of which can be represented by complex numbers, so that it is desirable to be able to derive the initial values of the complex coefficients directly from the training sequence involved.